This disclosure relates to double heterojunction nanoparticles, methods of manufacture thereof and to articles comprising the same.
One of the advantages of semiconductor nanocrystals is their potential for improving the efficiencies of optoelectronic devices. Spherical nanocrystal heterostructures, sometimes referred to as core-shell quantum dots, have been widely used for quantum dot light emitting diodes (LEDs). In these core-shell heterostructures which mainly consist of type I (straddling) band offset, the heterojunction serves merely as a passivation layer to improve photoluminescence efficiency. Owing to their unique optical and electronic properties, semiconductor nanocrystals have attracted much attention in various opto-electronic applications including photovoltaics (PVs), LEDs, solid state lighting, and displays. These tiny crystals have one or more dimensions that are a few nanometers in length, which allows tuning of their electronic band gap. The change in the band gap and the electronic energy levels allows one to control the observed optical and electrical properties of the semiconductor.
In addition, when two or more semiconductor materials are brought together, one can expect new and improved optical and electronic properties depending on their relative band offsets and band alignment. The heterojunction that is formed at the interface of dissimilar semiconductors can help to direct electrons and holes as well as being an active component for a variety of devices including PVs, LEDs and transistors. By choosing different materials for the core and the shell, the band edge positions may be varied. However, the effective band gap and the band offsets for some combinations of materials may be large and may hinder carrier injection processes. It is therefore desirable to produce semiconducting nanoparticles that have multiple heterojunctions. Particles having multiple heterojunctions allow the band gap and band offsets at different interfaces to be tuned by virtue of having more than two semiconducting materials selectively contacting one another.
Benefits of multiple heterojunctions include facilitating carrier injection and/or blocking while providing improved photoluminescence yields by surface passivation of the central “core”—that is, by creating multiple heterojunctions with a combination of type I and type II band offsets. This facilitates the achievement of a good barrier for one type of carrier while facilitating injection of the other carrier type in addition to the surface passivation benefits equivalent to type I core/shells.